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  <title>Document</title>
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<body>
  <!-- 有两个变量：Num1里面放的是10，num2里面放的是20
      最后变更为num1里面放的是20，num2里面放的是10
  -->

  <script>
    let num1 = 10
    let num2 = 20
    let num3
    num3 = num2
    num2 = num1
    num1 = num3
    document.write("num1的变更后为：" + num1)
    document.write("num2的变更后为：" + num2)
  </script>
</body>

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